The present invention relates to a terminal adapter pooling system for pooling a so-called terminal adapter so that the adapter is usable in common for a plurality of data terminals, where the adapter converts a digital interface provided for a conventional data terminal into a user-network interface for an ISDN (integrated service digital network).
The user-network interface for the ISDN is prescribed by a new standard (CCITT Recommendation 1,430/TTC Standard JT-1430). Therefore, it is necessary to provide the interface for the terminals in subscription to the ISDN. In other words, the conventional terminal provided with a V-series digital interface such as RS-232D cannot be directly connected to the ISDN, so that a terminal adapter (referred to as TA, hereinafter) is required as an additional apparatus for converting functions of the digital interface into other functions. By use of this TA, the user having the conventional data terminal such as a computer can enjoy the ISDN merits such as high speed and high reliable communications. This TA is also required when data terminals connected to key telephone sets or extension telephone sets of a PBX (private branch-exchange) via the RS-232D interface are connected to the ISDN. In this case, since it is not economical to provide a relatively high-costly TA for each data terminal, a single or several TAs are pooled, and the pooled TA is selectively connected to the data terminal only when the data terminal is required to communicate with the ISDN. In this specification, the above-mentioned system is referred to as a terminal adapter (TA) pooling system. In the system, conventionally, when a calling party number signal is transmitted from the data terminal or a called party number signal from the ISDN is received by the data terminal, a central control unit for controlling the call processing and the exchange operation transmits or receives control commands between the TA and the data terminal via the RS-232D interface, respectively, analyzes transmit/receive data to/from the RS-232D, and controls various call processing such as TA mode control, data terminal alerting, communication channel connection between the TA and the data terminal, etc. on the basis of the analyzed results. In parallel to the above processing, additionally, the communication channel connection between the TA and the ISDN is controlled. In this case, the TA automatically receives a called party number signal and transmits a calling party number signal in accordance with the procedure prescribed by V. 25 bis of CCITT Recommendation. In the above automatic operation, it is also required to finely control the sequence or the timing of the communication channel connection, for prevention of data emission.
In the conventional TA pooling system, since the processing executed by the central control unit at the calling party number signal transmission or the called party number signal reception is complicated, there exists a problem in that a large load is applied to the central control unit.